LTPS array substrate and method for producing the same

ABSTRACT

An LTPS array substrate and a method for producing the same are proposed. The method includes: forming an insulating layer, a semiconductor layer, and a first positive photoresist layer on the substrate one by one; exposing one side of the substrate on the opposite side of the gate for forming a polycrystalline silicon layer; forming a source and a drain of the TFT on the polycrystalline silicon layer; forming a pixel electrode on the insulating layer and part of the source; forming a plain passivation layer on a source-drain electrode layer; forming a transparent electrode layer on the plain passivation layer so that the transparent electrode layer is connected to the gate, the source, and the drain via the contact hole. The use of masks in types and numbers in the LTPS technology can be reduced. Thus, both of the processes and the production costs are reduced.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of co-pending patent application Ser.No. 14/760,750, filed on Jul. 14, 2015, which is a national stage of PCTapplication number PCT/CN2015/081634, filed on Jun. 17, 2015, claimingforeign priority of Chinese patent application number 201510310588.5,filed on Jun. 8, 2015.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to the field of display technology, andmore particularly, to a low temperature poly-silicon (LTPS) arraysubstrate and a method for producing the LTPS array substrate.

2. Description of Prior Art

Higher electron mobility is demonstrated in a liquid crystal display(LCD) adopting LTPS. The size of a thin film transistor (TFT) isactually smaller in this kind of LCD in which the aperture rate of apixel is higher, and the brightness is larger. Anyway, power consumptionand production costs are less using an LCD with LTPS. Owing to thesefeatures, the research on the LCD with LTPS is popular in LCDtechnology. However, the LTPS technology is sophisticated. It requires alot of masks in various types to fabricate an array substrate (arraysubstrate). Also, it needs many processes to produce an array substrate.Therefore, production costs are always high, which is a problem. To findways to reduce the masks used in the LTPS technology in numbers andtypes requires the whole industry to work out.

SUMMARY OF THE INVENTION

In view of this, an LTPS array substrate and a method for producing theLTPS array substrate is proposed by the embodiment of the presentinvention for the purpose of reducing the masks used in the LTPStechnology in types and in numbers.

According to a preferred embodiment of the present invention, a methodfor producing a low temperature poly-silicon (LTPS) array substratecomprises: forming a gate of a thin-film transistor (TFT) of the LTPSarray substrate on a substrate; forming a buffer layer on the substratewithout being covered by the gate and forming a plane using an uppersurface of the buffer layer and an upper surface of the gate; forming aninsulating layer, a semiconductor layer, and a first positivephotoresist layer on the substrate one by one in which an upper surfaceof the insulating layer is a plane; exposing one side of the substrateon an opposite of the gate for only preserving the first positivephotoresist layer disposed on a first section disposed right above thegate; injecting first impurity ions into the semiconductor layer outsidethe first section; exposing one side of the substrate on the oppositeside of the gate for forming the first positive photoresist layerdisposed on a second section disposed right above the gate, and thesecond section being smaller than the first section; injecting secondimpurity ions into the semiconductor layer outside the second section;removing the first positive photoresist layer disposed on the secondsection to form a polycrystalline silicon layer; forming a source and adrain of the TFT on the polycrystalline silicon layer; forming a pixelelectrode on the insulating layer and part of the source; forming aplain passivation layer on a source-drain electrode layer, which isfabricated from the source and the drain, and forming a contact via inthe plain passivation layer for exposing surfaces of the gate, thesource, and the drain, and the contact via being disposed outside thepolycrystalline silicon layer; forming a transparent electrode layer onthe plain passivation layer so that the transparent electrode layer canbe electrically connected to the gate, the source, and the drain via thecontact hole.

Furthermore, a step of forming the buffer layer on the substrate withoutbeing covered by the gate comprises: forming the buffer layer and anegative photoresist layer on the substrate one by one; exposing oneside of the substrate on the opposite side of the gate for removing thenegative photoresist layer disposed right above the gate; removing thebuffer layer disposed right above the gate, and preserving the bufferlayer on the substrate without being covered by the gate.

Furthermore, a step of forming the buffer layer on the substrate withoutbeing covered by the gate comprises: forming the buffer layer and asecond positive photoresist layer on the substrate one by one; exposingone side of the substrate facing the gate for removing the secondpositive photoresist layer disposed right above the gate; removing thebuffer layer disposed right above the gate, and preserving the bufferlayer on the substrate without being covered by the gate.

Furthermore, after removing the first positive photoresist layer on thesecond section, the method further comprises: coating a photoresistlayer on the polycrystalline silicon layer and exposing the photoresistlayer based on a desired pattern; etching the polycrystalline siliconlayer without the desired pattern; removing the remaining photoresistlayer.

Furthermore, the first impurity ions are N+ type impurity ions, and thesecond impurity ions are N− type impurity ions.

According to another embodiment of the present invention, a method forproducing a low temperature poly-silicon (LTPS) array substrate,comprises: forming a gate of a thin-film transistor (TFT) of the LTPSarray substrate on a substrate; forming an insulating layer, asemiconductor layer, and a first positive photoresist layer on thesubstrate one by one in which an upper surface of the insulating layeris a plane; exposing one side of the substrate on the opposite side ofthe gate for forming a polycrystalline silicon layer; forming a sourceand a drain of the TFT on the polycrystalline silicon layer; forming apixel electrode on the insulating layer and part of the source; forminga plain passivation layer on a source-drain electrode layer, which isfabricated from the source and the drain, and forming a contact via inthe plain passivation layer for exposing surfaces of the gate, thesource, and the drain, and the contact via being disposed outside thepolycrystalline silicon layer; forming a transparent electrode layer onthe plain passivation layer so that the transparent electrode layer canbe electrically connected to the gate, the source, and the drain via thecontact hole.

Furthermore, before the insulating layer is formed on the substrate ofthe gate, the method further comprises: forming a buffer layer on thesubstrate without being covered by the gate and forming a plane using anupper surface of the buffer layer and an upper surface of the gate.

Furthermore, a step of forming the buffer layer on the substrate withoutbeing covered by the gate comprises: forming the buffer layer and anegative photoresist layer on the substrate one by one; exposing oneside of the substrate on the opposite side of the gate for removing thenegative photoresist layer disposed right above the gate; removing thebuffer layer disposed right above the gate, and preserving the bufferlayer on the substrate without being covered by the gate.

Furthermore, a step of forming the buffer layer on the substrate withoutbeing covered by the gate comprises: forming the buffer layer and asecond positive photoresist layer on the substrate one by one; exposingone side of the substrate facing the gate for removing the secondpositive photoresist layer disposed right above the gate; removing thebuffer layer disposed right above the gate, and preserving the bufferlayer on the substrate without being covered by the gate.

Furthermore, a step of exposing one side of the substrate on theopposite side of the gate for forming a polycrystalline silicon layercomprises: exposing one side of the substrate on the opposite of thegate for only preserving the first positive photoresist layer disposedon a first section disposed right above the gate; injecting firstimpurity ions into the semiconductor layer outside the first section;exposing one side of the substrate on the opposite side of the gate forforming the first positive photoresist layer disposed on a secondsection disposed right above the gate, and the second section beingsmaller than the first section; injecting second impurity ions into thesemiconductor layer outside the second section; removing the firstpositive photoresist layer disposed on the second section.

Furthermore, after removing the first positive photoresist layer on thesecond section, the method further comprises: coating a photoresistlayer on the polycrystalline silicon layer and exposing the photoresistlayer based on a desired pattern; etching the polycrystalline siliconlayer without the desired pattern; removing the remaining photoresistlayer.

Furthermore, the first impurity ions are N+ type impurity ions, and thesecond impurity ions are N− type impurity ions.

Furthermore, steps of exposing one side of the substrate on the oppositeside of the gate for forming the polycrystalline silicon layer comprise:exposing one side of the substrate on the opposite side of the gate foronly preserving the first positive photoresist layer disposed on a firstsection disposed right above the gate; injecting P-type impurity ionsinto the semiconductor layer outside the first section; exposing oneside of the substrate on the opposite side of the gate for forming thefirst positive photoresist layer disposed on a second section disposedright above the gate, and the second section being smaller than thefirst section; removing the first positive photoresist layer disposed onthe second section.

According to another embodiment of the present invention, a lowtemperature poly-silicon (LTPS) array substrate, comprises: a substrate;a gate, disposed on the substrate; forming an insulating layer and apolycrystalline silicon layer on the substrate one by one in which anupper surface of the insulating layer is a plane; a source and a drain,disposed on the polycrystalline silicon layer; a pixel electrode,disposed on the insulating layer and part of the source; a plainpassivation layer, disposed on a source-drain electrode layer fabricatedfrom the source and the drain, and a contact via formed in the plainpassivation layer for exposing surfaces of the gate, the source, and thedrain, the contact via disposed outside the polycrystalline siliconlayer; a transparent electrode layer, disposed on the plain passivationlayer, and the transparent electrode layer being electrically connectedto the gate, the source, and the drain via the contact hole.

Furthermore, the LTPS array substrate further comprises a buffer layer,the buffer layer is disposed on the substrate without being covered bythe gate, and an upper surface of the buffer layer and an upper surfaceof the gate form a plane.

The fact about the LTPS array substrate and the method for producing theLTPS array substrate proposed by the present invention is that one sideof a substrate on the opposite side of a gate is exposed for forming apolycrystalline silicon layer. In other words, an opaque gate is exposedfor forming a polycrystalline silicon layer. It is unnecessary to useany masks in producing polycrystalline silicon layers so the use ofmasks in types and in numbers in the LTPS technology will be reduced.So, both of the processes and the production costs are reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of a method for producing an LTPS array substrateaccording to a preferred embodiment of the present invention.

FIG. 2 shows a process of forming a gate according to the presentinvention.

FIG. 3 shows a process of forming an insulating layer, a semiconductorlayer, and a positive photoresist formed on the substrate according tothe present invention.

FIG. 4 shows a process of forming a polycrystalline silicon layeraccording to the present invention.

FIG. 5 shows a patterned polycrystalline silicon layer according to thepresent invention.

FIG. 6 shows a process of forming a source and a drain according to thepresent invention.

FIG. 7 shows a process of forming a pixel electrode according to thepresent invention.

FIG. 8 shows a first cross-sectional view of forming a transparentelectrode according to the present invention.

FIG. 9 shows a process of forming a passivation layer according to thepresent invention.

FIG. 10 shows a second cross-sectional view of forming a transparentelectrode according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The drawings illustrate embodiments of the invention and, together withthe description, serve to explain the principles of the invention.

FIG. 1 is a flow chart of a method for producing an LTPS array substrateaccording to a preferred embodiment of the present invention. The methodcomprises the following steps:

Step 11: forming a gate of a thin-film transistor (TFT) of the LTPSarray substrate on a substrate.

Referring to FIG. 2, the substrate 21 is used for forming the LTPS arraysubstrate of the LCD panel. The substrate 21 can be a glass substrate, aplastic substrate, or a flexible substrate.

A first metallic layer is formed on the substrate 21 and the firstmetallic layer is exposed through a first mask in this embodiment. Theexposed first metallic layer is patterned after being developed andetched. Then, a gate 22 is formed. The first metallic layer is etchedwith etching liquid comprising phosphoric acid, nitric acid, aceticacid, and deionized water. Definitely, the first metallic layer canundergo dry etching as well.

The gate 22 can be acquired through other methods as well, such aschemical vapor deposition (CVD), plasma enhanced chemical vapordeposition (PECVD), sputtering, vacuum evaporation deposition, and lowpressure chemical vapor deposition (LP-CVD). The method of deposition isnot confined in the specification. The gate 22 with a desired pattern isformed on the substrate 21 directly. The first metallic layer isfabricated from a metal, such as aluminum (AL), molybdenum (Mo),titanium (Ti), chromium (Cr), and cuprum (Cu). Or, the first metalliclayer is fabricated from a metallic oxide, such as titanium oxide, ormetallic alloy or other conducting materials.

Step 12: forming an insulating layer, a semiconductor layer, and apositive photoresist layer on the substrate one by one in which theupper surface of the insulating layer is a plane.

Referring to FIG. 3 as well, a buffer layer 23 needs to be formed on thesubstrate 21 without being covered by the gate 22 before the insulatinglayer 25, the semiconductor layer 26, and the positive photoresist layer27 are formed in this embodiment. The concrete processes are describedbelow while it does not mean that the processes are limited.

Firstly, the buffer layer 23 and a negative photoresist layer 24 areformed on the substrate 21 of the gate 22 one by one. The buffer layer23 can be a SiN_(x) layer, a SiO_(x) layer or the combination of othernon-conducting materials. The buffer layer 23 is used for preventingupward spreading of the impurity in the substrate 21 in the followingprocesses so that the quality of a LTPS layer which will be formed inthe following processes will not be affected. The SiN_(x) layer and theSiO_(x) layer are deposited through CVD, PECVD, sputtering, vacuumevaporation deposition, or LP-CVD. The method of deposition is notconfined in the specification.

Next, one side of the substrate 21 on the opposite side of the gate 22is exposed. The negative photoresist layer 24 disposed right above thegate 22 is not exposed under the shelter of the gate 22 so the negativephotoresist layer 24 can be removed with ash after being developed.

Finally, the remaining negative photoresist layer 24 is lifted off. Thebuffer layer 23 disposed right above the gate 22 is etched and removed.Therefore, the buffer layer 23 without being covered by the gate 22 ispreserved.

The present embodiment further proposes a method for forming the bufferlayer 23. It is as follows:

Firstly, the buffer layer 23, the positive photoresist layer, thepositive photoresist layer 27 relatively formed on the semiconductorlayer 26 are formed on the substrate 21 of the gate 22 one by one. Thepositive photoresist layer can be understood as the second positivephotoresist layer. The positive photoresist layer 27 is the firstpositive photoresist layer. Then, one side of the substrate 21 facingthe gate 22 is exposed to remove the positive photoresist layer disposedright above the gate 22. Finally, the buffer layer 23 disposed rightabove the substrate 21 is removed. The buffer layer 23 on the substratewithout being covered by the gate 22 is preserved.

Step 13: exposing one side of the substrate on the opposite side of thegate for forming a polycrystalline silicon layer.

Referring to FIG. 4 as well, firstly, one side of the substrate 21 onthe opposite side of the gate 22 is exposed. The positive photoresistlayer 27 disposed on the first section Q₁ of the gate 22 is not exposedunder the shelter of the gate 22 so the positive photoresist layer 27 ispreserved after being developed. The positive photoresist layer 27without being exposed under the shelter of the gate 22 can be removedwith ash after being developed. Therefore, only the positive photoresistlayer 27 disposed on the first section Q₁ disposed right above the gate22 is preserved.

Next, first impurity ions are injected into the semiconductor layer 26outside the first section Q₁. In other words, conventionally, thesemiconductor layer 26 is heavily doped.

Next, one side of the substrate 21 on the opposite side of the gate 22is exposed. The intensity of this exposure is larger than the intensityof exposure on the positive photoresist layer 27 formed on the firstsection Q₁. So the positive photoresist layer 27 disposed on both sidesof the first section Q₁ is removed. The positive photoresist layer 27 ofthe second section Q₂ is formed right above the gate 22. The secondsection Q₂ is smaller than the first section Q₁.

Further, second impurity ions are injected into the semiconductor layer26 outside the second section Q₂. In other words, conventionally, thesemiconductor layer 26 is softly doped.

Finally, the positive photoresist layer 27 on the second section Q₂ isremoved.

The first impurity ions in this embodiment can be N+ type impurity ions,and correspondingly, the second impurity ions can be N− type impurityions. While the first impurity ions are P+ type impurity ions, thedoping of the second impurity ion is unnecessary. In other words, softdoping is unnecessary.

After the positive photoresist layer 27 is removed from the secondsection Q₂, a polycrystalline silicon layer 28 is exposed through asecond mask for forming a desired pattern on the polycrystalline siliconlayer 28. Referring to FIG. 5 as well, a photoresist layer 29 is coatedon the polycrystalline silicon layer 28 and exposed. Then, thepolycrystalline silicon layer 28 where no desired pattern is formed isetched and removed. Meanwhile, the remaining photoresist layer 29 isremoved as well. One side of the substrate 21 on the opposite side ofthe gate 22 is exposed when the photoresist layer 29 is a positivephotoresist layer. One side of the substrate 21 facing the gate 22 isexposed when the photoresist layer 29 is a negative photoresist layer.

Step S14: forming a source and a drain of the TFT on the polycrystallinesilicon layer.

A source S and a drain D of the TFT are formed through exposure,development, and etching through a third mask in this embodiment, asshown in FIG. 6.

Step S15: forming a pixel electrode on the insulating layer and part ofthe source.

A pixel electrode 30 with a desired pattern is formed through exposure,development, and etching through a fourth mask in this embodiment, asshown in FIG. 7.

Step S16: forming a plain passivation layer on a source-drain electrodelayer, which is fabricated from the source and the drain, and forming acontact via in the plain passivation layer for exposing the surfaces ofthe gate, the source, and the drain; the contact via being disposedoutside the polycrystalline silicon layer.

Step S17: forming a transparent electrode layer on the plain passivationlayer so that the transparent electrode layer can be electricallyconnected to the gate, the source, and the drain via the contact hole.

The plain passivation layer 31 is formed through exposure, development,and etching through a fifth mask in this embodiment, as shown in FIGS. 8and 9. Referring to FIG. 8, outside the TFT, the plain passivation layer31 comprises a contact via O. Because of the contact via O, the surfacesof the gate 22, the source S, and the drain D are exposed andelectrically connected to wires of the LTPS array substrate. Forexample, the gate 22 is correspondingly electrically connected to a gatewire formed on the substrate 21 (array substrate). The source S iscorrespondingly electrically connected to a data wire formed on thearray substrate. The gate wire and the data wire are vertically crossedand form a pixel display section where the pixel electrode 33 isdisposed.

A transparent electrode layer 32 is formed through exposure,development, and etching through a sixth mask in this embodiment, asshown in FIG. 10. The transparent electrode layer 32 and the pixelelectrode 30 are fabricated from the same transparent conductingmaterials and used as a common electrode of the LTPS array substrate.

One side of the substrate 21 on the opposite side of the gate 22 isexposed. In other words, the opaque gate 22 is exposed for forming thepolycrystalline silicon layer 28 without using any masks. Therefore, thetype and number of masks for producing the LTPS array panel is reduced.It not only simplifies the manufacture procedure but also reduces costs.

The present invention further proposes an LCD panel comprising the LTPSarray panel as shown in FIG. 10 and an LCD. The benefit is as what isdescribed above.

The present disclosure is described in detail in accordance with theabove contents with the specific preferred examples. However, thispresent disclosure is not limited to the specific examples. For theordinary technical personnel of the technical field of the presentdisclosure, on the premise of keeping the conception of the presentdisclosure, the technical personnel can also make simple deductions orreplacements, and all of which should be considered to belong to theprotection scope of the present disclosure

What is claimed is:
 1. A method for producing a low temperaturepoly-silicon (LTPS) array substrate, comprising: forming a gate of athin-film transistor (TFT) on a substrate; forming an insulating layer,a semiconductor layer, and a first positive photoresist layer on thesubstrate one by one in which an upper surface of the insulating layeris a plane; exposing one side of the substrate on the opposite side ofthe gate for forming a polycrystalline silicon layer; forming a sourceand a drain of the TFT on the polycrystalline silicon layer; forming apixel electrode on the insulating layer and a part of the drain; forminga plain passivation layer on a source-drain electrode layer, which isfabricated from the source and the drain, and forming contact vias inthe plain passivation layer for exposing surfaces of the gate and thedrain, and the contact vias being disposed outside the polycrystallinesilicon layer; and forming a transparent electrode layer on the plainpassivation layer so that the transparent electrode layer iselectrically connected to the gate via the contact via; wherein beforethe insulating layer is formed on the substrate, a pre-operation iscarried out to form a buffer layer on a portion of the substrate that isnot covered by the gate such that an upper surface of the buffer layerand an upper surface of the gate collectively form a plane, and thepre-operation comprises the following steps: forming the buffer layerand a negative photoresist layer on the substrate in sequence; exposingone side of the substrate on the opposite side of the gate for removinga portion of the negative photoresist layer disposed right above thegate; and removing the buffer layer disposed right above the gate suchthat a portion of the buffer layer is preserved on the portion of thesubstrate that is not covered by the gate; and wherein the step of“exposing one side of the substrate on the opposite side of the gate forforming a polycrystalline silicon layer” comprises the followingsub-steps: exposing one side of the substrate on the opposite side ofthe gate for only preserving the first positive photoresist layerdisposed on a first section disposed right above the gate; injectingP-type impurity ions into the semiconductor layer outside the firstsection; exposing one side of the substrate on the opposite side of thegate for forming the first positive photoresist layer disposed on asecond section disposed right above the gate, and the second sectionbeing smaller than the first section; and removing the first positivephotoresist layer disposed on the second section.
 2. The method asclaimed in claim 1, wherein the gate is formed through direct depositionon the substrate.
 3. The method as claimed in claim 1, wherein thebuffer layer is formed of a material comprising one of silicon nitrideand silicon oxide.
 4. The method as claimed in claim 1, wherein the gateis formed by first forming a first metallic layer on the substrate andthen patterning the first metallic layer.
 5. The method as claimed inclaim 4, wherein the step of patterning the first metallic layercomprises etching the first metallic layer with an etching liquidcomprising phosphoric acid, nitric acid, acetic acid, and deionizedwater.
 6. The method as claimed in claim 4, wherein the first metalliclayer is formed of a conducting material, which comprises one of ametal, a metal oxide, and a metal alloy.
 7. The method as claimed inclaim 6, wherein the metal comprises one of aluminum, molybdenum,titanium, chromium, and copper.
 8. The method as claimed in claim 6,wherein the metal oxide comprises titanium oxide.
 9. A method forproducing a low temperature poly-silicon (LTPS) array substrate,comprising: forming a gate of a thin-film transistor (TFT) on asubstrate; forming an insulating layer, a semiconductor layer, and afirst positive photoresist layer on the substrate one by one in which anupper surface of the insulating layer is a plane; exposing one side ofthe substrate on the opposite side of the gate for forming apolycrystalline silicon layer; forming a source and a drain of the TFTon the polycrystalline silicon layer; forming a pixel electrode on theinsulating layer and a part of the drain; forming a plain passivationlayer on a source-drain electrode layer, which is fabricated from thesource and the drain, and forming contact vias in the plain passivationlayer for exposing surfaces of the gate and the drain, and the contactvias being disposed outside the polycrystalline silicon layer; andforming a transparent electrode layer on the plain passivation layer sothat the transparent electrode layer is electrically connected to thegate via the contact via; wherein before the insulating layer is formedon the substrate, a pre-operation is carried out to form a buffer layeron a portion of the substrate that is not covered by the gate such thatan upper surface of the buffer layer and an upper surface of the gatecollectively form a plane, and the pre-operation comprises the followingsteps: forming the buffer layer a second positive photoresist layer onthe substrate in sequence; exposing one side of the substrate facing thegate for removing a portion of the second positive photoresist layerdisposed right above the gate; and removing the buffer layer disposedright above the gate such that a portion of the buffer layer ispreserved on the portion of the substrate that is not covered by thegate; and wherein the step of “exposing one side of the substrate on theopposite side of the gate for forming a polycrystalline silicon layer”comprises the following sub-steps: exposing one side of the substrate onthe opposite side of the gate for only preserving the first positivephotoresist layer disposed on a first section disposed right above thegate; injecting P-type impurity ions into the semiconductor layeroutside the first section; exposing one side of the substrate on theopposite side of the gate for forming the first positive photoresistlayer disposed on a second section disposed right above the gate, andthe second section being smaller than the first section; and removingthe first positive photoresist layer disposed on the second section. 10.The method as claimed in claim 9, wherein the gate is formed throughdirect deposition on the substrate.
 11. The method as claimed in claim9, wherein the buffer layer is formed of a material comprising one ofsilicon nitride and silicon oxide.
 12. The method as claimed in claim 9,wherein the gate is formed by first forming a first metallic layer onthe substrate and then patterning the first metallic layer.
 13. Themethod as claimed in claim 12, wherein the step of patterning the firstmetallic layer comprises etching the first metallic layer with anetching liquid comprising phosphoric acid, nitric acid, acetic acid, anddeionized water.
 14. The method as claimed in claim 12, wherein thefirst metallic layer is formed of a conducting material, which comprisesone of a metal, a metal oxide, and a metal alloy.
 15. The method asclaimed in claim 14, wherein the metal comprises one of aluminum,molybdenum, titanium, chromium, and copper.
 16. The method as claimed inclaim 14, wherein the metal oxide comprises titanium oxide.